The invention relates to a method and apparatus for monitoring the formation of deposits caused by depositions of solid particles from a hot, dust-laden flue gas onto the walls of a rectangular furnace of a boiler by taking an infrared image of the walls with the aid of an infrared camera, wherein the walls are formed by tubes that are tightly welded together and through which a cooling medium flows.
When boilers are fired with solid fuel, on the flue gas side deposits are formed on the heating surfaces due to the deposition of solid particles, for example ash. Due to their heat-insulating effect, such deposits on the heating surfaces hinder the transfer of heat from the flue gas to the working medium (water/water vapor) in the tubular walls of the heating surfaces, thus reducing the efficiency of the boiler.
In the region of the furnace, the deposits are cleaned off by means of high pressure water streams of water or water lance blowers. In this connection, it is desired on the one hand to clean off the deposits as completely as possible, and on the other hand to prevent the water stream from striking clean heating surface regions. The latter leads to an unnecessary stressing of the material of the tubular walls of the heating surfaces as a consequence of thermal shock and the thereby resulting damage to the boiler. A further desire is to have to clean only as often as necessary in order to avoid efficiency losses due to the cleaning process. To control the cleaning devices in the furnace, the following methods are used pursuant to the present state of the art:
a) Time Control:
Based on experience data, the entire furnace is cleaned after fixed time intervals have elapsed. In this connection, neither the deposits that are formed are attacked in a selective manner nor are areas that have remained cleaned spared.
b) Heat-Type Diagnosis of the Heat Transfer Capability of the Heating Surfaces:
By measuring entry and exit parameters of the working medium, the reduction of the heat transfer of the heating surfaces is diagnosed and the cleaning process is initiated. The heating surfaces arranged in the furnace belong to the most part to the evaporator, which from a thermal standpoint can only be diagnosed as a whole. Thus, cleaning of the entire evaporator heating surface is always initiated, without sparing clean areas.
c) Localization of Deposits via Heat-Flux Density Probes Welded into the Heating Surfaces:
The heat-flux from the flue gas to the working medium is measured in a point-focal manner, and the heating surfaces are cleaned by sections based upon the measured data. This enables contaminated regions to be cleaned in a selective manner while sparing clean regions. However, the installation and maintenance of the heat-flux density probes is very complicated and expensive. Therefore, only few measurement locations are installed, so that each measurement point involves several hundred square meters of heating surface. Thus, it is not possible to ensure that the point-focal measurement is representative of the associated heating surface region, i.e. the predominant portion of the region can, for example, be clean, although the point-type measurement indicates contamination.
d) Localization of Deposits with Infrared Camera Systems:
It is known to use infrared camera systems to evaluate the degree of contamination of heating surfaces, and by computer-supported evaluation of the infrared images to determine the geometrical extent of the deposits (DE 195 47 269 A1). In accordance with an evaluation, the deposits are removed by a shock generator. To carry out the known method, the infrared cameras are disposed in hatches and inspection flaps of the flue for flue gas that is disposed downstream of the furnace and accommodates contact heating surfaces. No details are provided in DE 195 47 269 A1 about the configuration of the infrared cameras and the evaluation of the measurement results.
With the method known from DE 41 39 738 C2, an infrared image of the walls of the furnace of a boiler is taken with the aid of an infrared camera. The infrared camera that is utilized operates in the near infrared range at a wavelength of from 1.5 to 2.1 μm. The known method can be used only for ash deposits having a high degree of reflection. The method furthermore presupposes a reference region on the furnace wall that is not to be cleaned. The intensity ratio between the region that is to be cleaned and the reference region is the measure for the contamination of the region that is to be cleaned. Thus, it is not possible to completely clean the entire wall.
It is an object of the invention to make the monitoring of the formation of deposits upon the walls of furnaces with the aid of infrared cameras simpler and universally usable.